An industrial production process of acrylic acid to be widely used currently is mainly composed of (i) catalytic gas-phase oxidation step, (ii) collection step and/or condensation step, and (iii) purification step.
(i) In the catalytic gas-phase oxidation step, acrylic acid is synthesized by catalytic gas-phase oxidation of raw materials of acrylic acid such as propane, propylene and acrolein with a molecular oxygen-containing gas in the presence of an oxidation catalyst. In this case, a mixed gas which contains acrylic acid of desired product and byproducts or impurities (hereinafter, also simply called as “impurities etc.”) such as acetic acid, formaldehyde, and acrylic acid dimer is obtained as a reaction product.
(ii) In the collecting step and/or condensation step, the mixed gas containing an acrylic acid and impurities etc. obtained at the above catalytic gas-phase oxidation step is recovered in liquid state. For example, in the collecting step, an acrylic acid-containing solution containing an acrylic acid and impurities etc. is obtained by introducing the above mixed gas into a collecting tower, and collecting the gas by contacting with a collecting solvent such as water.
(iii) In the purification step, a purified acrylic acid in high purity is obtained by purifying the acrylic acid-containing solution obtained at the above collecting step and/or condensation step by a purification means such as distillation step and/or crystallization step. For example, in the crystallization step, firstly acrylic acid is crystallized by cooling the acrylic acid-containing solution, and the mother liquid is recovered. Subsequently, the crystal of acrylic acid is melted to obtain acrylic acid having high purity. It should be noted that, the term “mother liquid” as used herein means a solution that remains after depositing the crystal of acrylic acid from the acrylic acid-containing solution.
A crystallization unit to be used in such crystallization step generally has a crystallizer, a supply means to supply the acrylic acid-containing solution into the crystallizer, and a recovery means to recover the mother liquid or the purified acrylic acid.
For example, Patent Literature 1 discloses a crystallization method using a multistage fractional crystallization method. And, as a crystallization unit to be used for the method, a crystallization unit having piping equipped with valves therein to transport melted liquid of the crystal and the mother liquid obtained at the crystallization unit into a tank has been disclosed.
In addition, Patent Literature 2 discloses a crystallization method in which, when crystal is melted, a polymerization inhibitor is added to a heated purified melted liquid, and this solution is circulated and supplied to crystal to melt the crystal, and both melted liquid from crystal and purified melted liquid are recovered. Also, a unit has been disclosed in which the melted liquid from crystal and the mother liquid from the crystallization unit are transported to tanks through piping equipped with valves therein, when the melted liquid from crystal and the melted purified liquid containing a polymerization inhibitor added thereto are recovered.